1. Field of the Invention
The present invention relates to an antenna coil to be mounted on a circuit board for use in an RFID (radio frequency identification) system that performs communication with an external apparatus via an electromagnetic signal, and also relates to an antenna device including such an antenna coil.
2. Description of the Related Art
In RFID systems that have recently been in increasing use, an antenna for information communication is mounted in each of a mobile electronic device, such as a mobile phone, and a reader/writer so that data is exchanged between the mobile electronic device and the reader/writer. In particular, there is a strong demand for an antenna mounted in a mobile electronic device to achieve high performance, low cost, and small size. In order to meet this demand, an antenna coil is used.
For example, Patent Document 1 (Japanese Unexamined Patent Application Publication No. H11-122146), discloses an antenna mounted in a mobile electronic device. FIG. 17 is a perspective view showing a configuration of the antenna device described in Patent Document 1. A coil that forms an information communication antenna 102 mounted on a board 101 includes a plurality of segments 102a and 102b. Each segment includes a magnetic core and a coil wound around the magnetic core. The coil of the first segment 102a is wound left-handed, and the coil of the second segment 102b is wound right-handed. The coil of the first segment 102a and the coil of the second segment 102b are connected to each other. A portion where a coil conductor is not provided (hereinafter referred to as a non-winding portion) is provided between the segments 102a and 102b. When the antenna coil 102 is mounted in this way, a magnetic flux that is perpendicular to the board is bent about 90° after entering the non-winding portion, and is then guided to the first segment 102a and the second segment 102b. When the magnetic flux passes through the coil axes of the coils of the segments 102a and 102b, voltages are induced in the coils, and communication is allowed.
The above-described antenna coil 102 functions as an antenna because magnetic flux entering the coil-conductor non-winding portion is guided to the segments 102a and 102b. If the non-winding portion is small, a sufficient magnetic flux cannot be captured. In contrast, if the non-winding portion is too large, the magnetic flux is not guided to the segments 102a and 102b. In each case, the magnetic flux does not pass through the coil axes of the coils of the segments 102a and 102b, and electromagnetic induction does not occur. Therefore, the segments 102a and 102b need to be arranged with a fixed space therebetween.
Unfortunately, in the configuration described in Patent Document 1, when the antenna coil 102 is mounted on the board 101 of the mobile electronic device, the segments 102a and 102b that constitute the antenna coil 102 are fixed separately. For this reason, it is necessary to finely adjust the fixing positions so that the distance between the segments is fixed. This adjustment needs many steps. Further, when the distance between the segments varies in accordance with the fixing positions, an expected antenna sensitivity is not achieved, depending on the structure of the mobile electronic device in which the antenna is mounted.